There are various problems.. but the main one would be.. they all have different content (though i don't know how the xbox version is versus the playstation one.. but they're definitely different from the PC version)

To answer your question, the PC and Console versions are built differently, software wise, and all contain exclusive or yet to be introduced content (Xbox WoT has the T-34-88, and exclusive maps, PC has AMX 13-57, unreleased content, updated game mechanics, etc). It would take a substantial amount of work to get all three versions on equal footing software wise, and then WG would have to decide whether or not to give each version access to all exclusive features or not, which would be disasterous business wise.

Plus, like most have said, the PC community is significantly more skilled at the game than either of the console communities are. A server merge would lessen overall enjoyment for everybody playing, as steamrolls would be even more frequent and even more dramatic. I've experienced skill deficit first hand, and it's ridiculous how large the skill gap is. It's like a x5 weekend on crack.

Aso, iirc WoT console is run by a different dev studio than WoT PC is, so there's that to.

Theoretically it can be done, one of the other tank games has done it, but as of now it would take too much work, and would lose WG too much money for them to merge console and PC servers.